$\begingroup$Are you asking about meiosis or mitosis?$\endgroup$
– MattDMoApr 1 '15 at 20:34

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$\begingroup$Welcome to SE.biology! These papers 1, 2 should help you. Don't hesitate to come back with more specific questions if you need to. We are happy to answer them.$\endgroup$
– cagliari2005Apr 1 '15 at 23:15

$\begingroup$If your textbook only says "they line up", then you should get a better textbook.$\endgroup$
– fileunderwaterApr 2 '15 at 7:45

$\begingroup$It is unreasonable to close this as a homework question.$\endgroup$
– The Last WordApr 7 '15 at 4:18

1 Answer
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There are many things involved in the pairing of homologue chromosomes.

Before mitosis can occur an important prerequisite must happen: the division of centrosome. This small complex is the principal microtubule-organizing center in the (animal and therefore human) cells. During interphase the microtubules originating from the centrosome, project to the cell perimiter with their + ends (this where they grow). At the beginning of mitosis the duplicated centrosomes separate and migrate to oppsite sites of the nucleus to form the poles of the mitotic spindle. As the nuclear envelope disintegrates the spindle captures the chromosomes at the cetromeres. Since the microtubules are growing from the centrosomes this capture eventually pushes the choromosomes to center. Source: The Molecular Biology of the cell. Fourth Edition. ISBN: 0-8153-3218-1 (hardbound) 0-8153-4072-9 - (pbk).
A searchable online version: http://www.ncbi.nlm.nih.gov/books/NBK21054/

$\begingroup$We need to make a distinction between the pairing of homologous chromosomes that occurs during meiotic prophase--the pairing that is necessary for homologous recombination, and that forms crossovers, or chiasmata--vs. the lining up of mother and daughter chromosomes on the mitotic spindle's metaphase plate after DNA replication (during S-phase of the cell-cycle). So yes, to ensure that each daughter of a dividing mitotic cell gets one pair of every chromosome, a requisite step is to build a mitotic spindle, and so you need to duplicate the centrosome (MTOC) first.$\endgroup$
– mdperryApr 4 '15 at 2:39

$\begingroup$In one of your dividing somatic cells you have 2 copies of chromosome 7, one from your mother, and one from your father. After DNA replication and cell division, each daughter cell will also have 2 copies of chromosome 7, one of each type. The daughter cells never get 2 copies of the father's chr 7 (nor 2 copies of the mother's chr 7). In other words, there is no segregation where the genetic constitution of one daughter cell differs from the other daughter cell, the cells are identical. The two replicated father's chromosomes on the mitotic spindle are not paired along their lengths.$\endgroup$
– mdperryApr 4 '15 at 2:46

$\begingroup$Their centromeres are attached to each other, and to the spindle. More specifically, the two copies of the father's chr 7 may not be anywhere near the two copies of the mother's chr 7. Pairing of chromosomes during interphase is not particularly common. In dipterans, there are some tissues, such as the larval salivary glands, where successive rounds of endoreduplication lead to polyploid cells with giant aligned chromosomes, also called polytene chromosomes, but this is a specialized situation. The gut nuclei of nematodes also endoreduplicate, without forming these lined up daughters.$\endgroup$
– mdperryApr 4 '15 at 2:56